Failure detecting apparatus for exhaust secondary air supply system

ABSTRACT

In a V-type engine in which two air supply pipes are connected to banks thereof, respectively, two air-fuel ratio sensors are provided at positions downstream of the air supply pipes for producing outputs corresponding to the density of oxygen in exhaust emissions. When exhaust secondary air is supplied, differences between air-fuel ratio feedback correction factors calculated based on outputs of two air-fuel ratio sensors at a time and air-fuel feedback correction factors calculated at a time that is a time that results after a predetermined period of time has elapsed since the time are calculated. The differences so calculated are compared with a predetermined value, and, when the calculated differences do not exceed the predetermined value, the exhaust secondary air supply system is detected to fail.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a failure detecting apparatusfor an exhaust secondary air supply system.

[0003] 2. Description of the Related Art

[0004] An exhaust secondary air supply system includes, for example, anair supply pipe connected to a position upstream of a catalyticconverter disposed along an exhaust system of an internal combustionengine and an air pump. The exhaust secondary air supply system isintended to introduce secondary air from the air supply pipe into theexhaust system by driving the air pump so as to promote the combustionin the system to thereby reduce unburned constituents contained inexhaust gases.

[0005] In the exhaust secondary air supply system, since, in case thereoccurs a failure such as a damage to the air supply pipe, the expectedfunction is not attained, various types of failure detecting methodshave been proposed therefor, and a technique described in JP-A-5-26033is disposed as an example.

[0006] In the technique described in the above-mentioned JP-A-5-26033,there is proposed an exhaust secondary air supply system including anair supply pipe connected to an air pump driven by an internalcombustion engine for supplying secondary air to an upstream position ofa catalytic converter and a control valve for controlling the opening ofthe air supply pipe so as to control the supply amount of secondary air.In this system, a failure of the exhaust secondary air supply system isdetected by supplying secondary air intermittently in a predetermineddiagnostic operating area to determine whether or not the output of anair-fuel ratio sensor (O2 sensor) disposed between an air supplyposition and the catalytic converter is reversed in response to theintermittent supplies.

[0007] In the related-art technique, in order to detect such a failureaccurately, an output of the air-fuel ratio sensor resulting when theexhaust secondary air supply system operates normally needs to beverified through learning, which is troublesome, and there has beencaused the inconvenience of the detection of a failure being delayed dueto the necessity of such verification. The inconvenience becomesnoticeable in particular, in the case of a V-type engine in which anexhaust system is provided for each bank.

SUMMARY OF THE INVENTION

[0008] Consequently, an object of the invention is to provide a failuredetecting apparatus for an exhaust secondary air supply system which canresolve the drawback described above and detect the failure of theexhaust secondary air supply system easily and accurately.

[0009] With a view to attaining the object, according to a first aspectof the invention, there is provided a failure detecting apparatus for anexhaust secondary air supply system for supplying exhaust secondary airto a position upstream of a catalytic converter disposed along anexhaust system of an internal combustion engine, the failure detectingapparatus comprising:

[0010] an air-fuel ratio sensor for producing an output corresponding toa density of oxygen contained in exhaust gas flowing through the exhaustsystem; and a failure detecting section for comparing values obtained atdifferent times based on outputs of the air-fuel ratio sensor when theexhaust secondary air is supplied so as to detect a failure of theexhaust secondary air supply system.

[0011] Since there is provided the failure detecting section forcomparing values obtained at different times based on outputs of theair-fuel ratio sensor when the exhaust secondary air is supplied so asto detect a failure of the exhaust secondary air supply system, thefailure of the exhaust secondary air supply system can be detectedeasily and accurately. In addition, since the necessity is obviated ofdoing the troublesome work in which the output of the air-fuel ratiosensor resulting when the exhaust secondary air supply system is normalis learned to be verified, there is no risk that the detection of thefailure of the exhaust secondary air supply system is delayed.Furthermore, sine the failure detection is not such as to be implementedbased on the output of the air-fuel ratio sensor when exhaust secondaryair is supplied, in other words, based on the output of the air-fuelratio sensor when the exhaust secondary air supply system is activatedintermittently so as to supply exhaust secondary air intermittently,there is no risk that an erroneous failure detection results from theeffect of an error caused by a difference in load of the internalcombustion engine.

[0012] According to a second aspect of the invention, there is provideda failure detecting apparatus for an exhaust secondary air supply systemas set forth in the first aspect of the invention, wherein the failuredetecting section comprises a correction factor difference calculatingsection for calculating a difference between an air-fuel feedbackcorrection factor calculated based on an output of the air-fuel ratiosensor at a time tn and an air-fuel feedback correction factorcalculated at a time tn+m that is a time that results after apredetermined period of time m has elapsed since the time tn and acomparing section for comparing the difference so calculated with apredetermined value, so as to determine that the exhaust secondary airsupply system fails when the calculated difference does not exceed thepredetermined value.

[0013] Since a difference between an air-fuel feedback correction factorcalculated based on an output of the air-fuel ratio sensor at a time tnand an air-fuel feedback correction factor calculated at a time tn+mthat is a time that results after a predetermined period of time m haselapsed since the time tn is calculated so as to be compared with apredetermined value, whereby, when the calculated difference does notexceed the predetermined value, the exhaust secondary air supply systemis detected to fail, the failure of the exhaust secondary air supplysystem can be detected more easily and accurately by setting thepredetermined period of time appropriately or to, for example, a timethat will be described below as a time needed for a change in theoperating conditions such as load of the internal combustion enginecaused by an activation of the exhaust secondary air supply system tocome to an end.

[0014] According to a third aspect of the invention, there is provided afailure detecting apparatus for an exhaust secondary air supply systemas set forth in the second aspect of the invention, wherein thepredetermined period of time is set based on a time needed for a changein operating conditions of the internal combustion engine caused by anactivation of the exhaust secondary air supply system to come to an end.

[0015] Since the predetermined period of time is set based on a timeneeded for a change in the operating conditions of the internalcombustion engine caused by an activation of the exhaust secondary airsupply system to come to an end, even when the air-fuel ratio sensorexhibits unexpected behaviors according to the change, an effectresulting from the unexpected behaviors can be avoided, whereby thefailure of the exhaust secondary air supply system can be detected moreeasily and accurately.

[0016] According to a fourth aspect of the invention, there is provideda failure detecting apparatus for an exhaust secondary air supply systemfor supplying exhaust secondary air to a position upstream of acatalytic converter disposed along an exhaust system of an internalcombustion engine, the failure detecting apparatus comprising:

[0017] a plurality of air supply pipes connected to the exhaust systemof the internal combustion engine for supplying the exhaust secondaryair, respectively;

[0018] a plurality of air-fuel ratio sensors disposed along the exhaustsystem at positions downstream of the plurality of air supply pipes forproducing an output corresponding to a density of oxygen contained inexhaust emissions flowing through the exhaust system, respectively; and

[0019] a failure detecting section for comparing values with each otherwhich are obtained based on the outputs of the plurality of air-fuelratio sensors so as to detect a failure of the exhaust secondary airsupply system.

[0020] Since there are provided the plurality of air-fuel ratio sensorsdisposed along the exhaust system at positions downstream of theplurality of air supply pipes for producing an output corresponding tothe density of oxygen contained in exhaust emissions flowing through theexhaust system, respectively, and the failure detecting section forcomparing values with each other which are obtained based on the outputsof the plurality of air-fuel ratio sensors so as to detect a failure ofthe exhaust secondary air supply system, the failure of the exhaustsecondary air supply system can be detected easily and accurately. Inaddition, the necessity is obviated of doing the troublesome work inwhich the output of the air-fuel ratio sensor resulting when the exhaustsecondary air supply system is normal is learned to be verified, andthere is caused no inconvenience that the detection of the failure ofthe exhaust secondary air supply system is delayed. In addition, in acase where an air supply pipe is provided for each exhaust system of anengine such as a V-type engine which has an exhaust system on each bank,it is possible not only to detect the existence of a failure but also todetect which air supply pipe on the banks suffers from the failure whenthe failure is detected.

[0021] According to a fifth aspect of the invention, there is provided afailure detecting apparatus for an exhaust secondary air supply systemas set forth in the fourth aspect of the invention, wherein the failuredetecting section comprises a correction factor difference calculatingsection for calculating a difference between air-fuel ratio feedbackcorrection factors calculated, respectively, based on the outputs of theplurality of air-fuel ratio sensors, and a comparing section forcomparing the difference so calculated with a predetermined value, so asto determine that the exhaust secondary air supply system fails when thecalculated difference exceeds the predetermined value.

[0022] Since the difference between the air-fuel ratio feedbackcorrection factors calculated, respectively, based on the outputs of theplurality of air-fuel ratio sensors are compared with the predeterminedvalue, whereby, when the calculated difference exceeds the predeterminedvalue, the exhaust secondary air supply system is detected to fail, thefailure of the exhaust secondary air supply system can be detected moreeasily and accurately.

[0023] According to a sixth aspect of the invention, there is provided afailure detecting apparatus for an exhaust secondary air supply systemas set forth in the fourth aspect of the invention, wherein the internalcombustion engine is a V-type engine, wherein the failure detectingapparatus comprises an air pump, and wherein the air supply pipe isconnected to the air pump and is branched at an intermediate positionalong its length so as to be connected to banks of the V-type engine,respectively.

[0024] According to construction, in a V-type engine in which an airsupply pipe is connected to each of banks thereof, the failure of anexhaust secondary air supply system can be detected easily andaccurately.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a schematic view illustrating the overall constructionof a failure detecting apparatus for an exhaust secondary air supplysystem according to an embodiment of the invention;

[0026]FIG. 2 is an explanatory perspective view showing in detail partof the apparatus shown in FIG. 1 including air supply pipes which areconstituent components of the exhaust secondary air supply system;

[0027]FIG. 3 is a flowchart of an operation of the apparatus shown inFIG. 1;

[0028]FIG. 4 is a time chart explaining the operation of the apparatusshown in FIG. 3 which shows outputs of air-fuel ratio sensors (O2sensors) resulting when no failure occurs in the exhaust secondary airsupply system and air-fuel ratio feedback correction factors that arecalculated based on the outputs;

[0029]FIG. 5 is a time chart explaining the operation of the apparatusshown in FIG. 3 which shows outputs of the air-fuel ratio sensors (O2sensors) resulting when a failure occurs in the exhaust secondary airsupply system and air-fuel ratio feedback correction factors that arecalculated based on the outputs;

[0030]FIG. 6 is a flowchart, which is similar to that shown in FIG. 3,illustrating an operation of a failure detecting apparatus for anexhaust secondary air supply system according to a second embodiment ofthe invention; and

[0031]FIG. 7 is a flowchart of an operation of a failure detectingapparatus for an exhaust secondary air supply system according to athird embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Hereinafter, a failure detecting apparatus for an exhaustsecondary air supply system according to embodiments of the inventionwill be described by reference to the accompanying drawings.

[0033]FIG. 1 is a schematic view showing the overall structure of afailure detecting apparatus for an exhaust secondary air supply systemaccording to an embodiment of the invention.

[0034] In the same drawing, reference numeral 10 denotes amulti-cylinder internal combustion engine (hereinafter, referred to asan “engine”). The engine 10 is a four-cycle vee six-cylinder DOHC engineand has three cylinders 12 including cylinders numbers 1, 2 and 3 on aright bank 10R and three cylinders 12 including cylinders numbers 4, 5and 6 on a left bank 10L.

[0035] In the engine 10, air taken in from an air cleaner 14 flowsthrough an induction pipe 16 and reaches to intake ports of therespective cylinders via induction manifolds (not shown) while the flowrate of the air is being controlled by a throttle valve 20. Gasolinefuel is then injected from injectors (not shown) disposed at the intakeports. Thus, an air-fuel mixture so formed enters combustion chambers(not shown) of the respective cylinders when inlet valves (not shown)are opened and then burns when ignited by sparking plugs (not shown).

[0036] Exhaust emissions (exhaust gases) produced by combustion flowinto exhaust manifolds 22 provided, respectively, on the left and rightbanks 10R, 10L when exhaust valves (not shown) are opened, merge atcollecting portions, and thereafter flow through exhaust pipes 24 to theoutside of the engine after harmful constituents have been removed atcatalytic converters (of three-way type) 26.

[0037] An exhaust secondary air supply system 30 is provided at aposition along the length of the exhaust system made up of the exhaustmanifolds 22 and the exhaust pipes 24. The exhaust secondary air supplysystem 30 mainly includes an air supply pipe (a delivery pipe) 32connected to a position upstream of the catalytic converters 26 alongthe exhaust system of the engine 10 and an air pump 34.

[0038] The induction pipe 16 is branched on an upstream side of thethrottle valve 20, and a branch 16 a connects to an intake side of theair pump 34 at the other end thereof. The air pump 34 connects to theair supply pipe 32 on an outlet side thereof. The air supply pipe 32 isbranched via a cut-off valve 36 so as to be connected to the exhaustmanifolds 22 on the left and right banks 10R, 10L, respectively. The airsupply pipe disposed on the exhaust manifold 22 on the right bank 10Rside is denoted as 32R, whereas the air supply pipe disposed on theexhaust manifold 22 on the left bank 10L side is denoted as 32L. Notethat the air supply pipes 32R, 32L are constructed so as to haveconfigurations which supply the same amount of air.

[0039] While the construction of the air supply pipe 32 is described indetail in FIG. 2, as shown in the drawing, flanges 32 b are formed atdistal ends of the air supply pipe 32 in the vicinity of openings 32 a,whereby the air supply pipe 32 is connected to the exhaust manifolds 22by being bolted to the exhaust manifolds 22 at the flanges 32 b whilealigning the openings 32 a with holes (shown by broken lines in FIG. 1)22 a opened in the exhaust manifolds 22.

[0040] An electric motor 40 is connected to the air pump 34, whereby theair pump 34 is driven by virtue of the rotation of the electric motor 40so as to suck in air taken in from the air cleaner 14 to thereby sendair so sucked in to the air supply pipe 32 under pressure. The cut-offvalve 36 includes a negative pressure diaphragm (not shown), so that thevalve opens when a negative pressure is introduced via a negativepressure introducing mechanism, not shown, so as to supply pressurizedair that has been introduced from an inlet opening 36 a to the exhaustmanifolds 22.

[0041] In FIG. 1, a crank angle sensor 42 is disposed in the vicinity ofa rotational shaft (not shown) such as a crankshaft of the engine 10 foroutputting a cylinder identifying signal, and the crank angle sensor 42also outputs a TDC signal at a TDC position or in the vicinity thereofof each cylinder and a crank angle signal which results from thesegmentation of the crank angle signal.

[0042] In addition, an absolute pressure sensor 44 is provideddownstream of a position along the induction pipe 16 where the throttlevalve 20 is disposed for outputting a signal corresponding to aninduction manifold absolute pressure (an engine load) PBA, and a coolanttemperature sensor 46 is disposed along a coolant passageway (not shown)of the engine 10 for outputting a signal corresponding to the enginecoolant temperature TW.

[0043] Additionally, in the exhaust system, primary air-fuel ratiosensors 50 are disposed an upstream side of the catalytic converters 26and secondary air-fuel ratio sensors 52 are disposed on a downstreamside thereof, and the respective sensors output signals corresponding tothe density of oxygen contained in exhaust emissions which flow throughpositions where those sensors are disposed, respectively (hereinafter,the sensors 50, 52 disposed on the right bank 10R are denoted as 50R,52R, whereas the sensors disposed on the left bank 10L are denoted as50L, 52L). The primary and secondary sensors are both an O2 sensor andoutput a signal which repeats reversals toward a rich direction and alean direction across a value corresponding to a stoichiometric air-fuelratio. Hereinafter, the primary air-fuel sensor 50 is referred to as a“PO2 sensor” and the secondary air-fuel sensor 52 is referred to as a“S₀₂ sensor”.

[0044] Outputs of the group of sensors are sent to an ECU 54. The ECU 54is made up of a microcomputer and not only counts crank angle signalsinputted from the crank angle sensor 42 so as to detect an enginerotational speed NE but also calculates a fuel injection amount TI thatis to be supplied to the engine 10 based on outputs of the sensorsincluding the crank angle sensor 42 as will be expressed below.

TI=TIM×KO2×KTOTAL+TTOTAL

[0045] where, TIM is a base value that is obtained from the enginerotational speed NE and engine load (induction manifold absolutepressure)PBA through map retrieving. In addition, KO2 is an air-fuelratio feedback correction factor that is determined based on a detectedair-fuel ratio obtained from an output of the PO2 sensor and iscalculated as will be expressed below. Hereinafter, n is a sample numberof a discrete system or, to be more specific, a control cycle.

[0046] KO2(n)=KO2(n−1)-KO2I (in the case where a detected air-fuel ratiois rich);

[0047] KO2(n)=KO2(n−1)+KO2I (in the case where a detected air-fuel ratiois lean)

[0048] Namely, KO2 is determined by adding or subtracting the I (anintegrating control term) to or from a deviation from a valuecorresponding to a stoichiometric air-fuel ratio (a center value betweenreversals). Note that KO2 is calculated for each bank based on outputsof the PO2 sensors 50R, 50L disposed on the left and right banks 10R,10L, respectively. In addition, KO2 is controlled to be learned.

[0049] Additionally, KTOTAL is a correction factor in anothermultiplying format, and TTOTAL is a correction factor in an addingformat. In addition, the fuel injection amount TI is shown as a valveopening time of the injector. Furthermore, the fuel injection amount TIis increased, for example, when the engine 10 is started.

[0050] The ECU 54 determines an ignition timing using the enginerotational speed NE and gives an instruction to energize the electricmotor 40 for a predetermined period after the engine 10 has been startedso as to drive the air pump 34 to supply exhaust secondary air to theexhaust system. Therefore, unburned constituents of fuel the amount ofwhich is increased when the engine 10 is started are burned in theexhaust manifolds 22 and the exhaust pipes 24 downstream of the exhaustmanifolds 22 to thereby be discharged into the atmosphere while heatingthe catalytic converters 26. Thus, the activation of the catalyticconverters 26 is promoted and the discharge of unburned constituents tothe atmosphere is reduced. In addition, the ECU 54 also detects thefailure of the exhaust secondary air supply system 30.

[0051] Next, the operation of detecting the failure of the exhaustsecondary air supply system 30 will be described.

[0052]FIG. 3 is a flowchart illustrating the operation.

[0053] To start the description of the flowchart, in step S10, whetheror not it is in a monitor area (a failure detecting area) is determined.It is determined to be in a monitor area when the engine 10 is in theidling state or other steady-state operating conditions after the engine10 is started and the warming up thereof is completed.

[0054] If denied in step 10, processes in the following steps areskipped, whereas if acknowledged, the flow proceeds to step S12, wherewhether or not the failure of the exhaust secondary air supply system 30has been detected is determined. If acknowledged in step S12, too,processes in the following steps are skipped. Note that the processes inthe following steps are also skipped, for example, if it is determinedthat the electric motor 40 does not fail but is heated excessively dueto the excessive energization of the electric motor 40.

[0055] On the other hand, if denied in step S12, the flow proceeds toS14, where the air pump 34 is switched on, or the electric motor 40 isenergized so as to activate the air pump 34, and the learning of theair-fuel feedback correction factor KO2 is prohibited. Namely, in ordernot to effect the originally desired air-fuel ratio feedback controlthrough the artificial operation of air-fuel ratio for failuredetection, the learning thereof is prohibited.

[0056] Next, the flow proceeds to S16, where, after a certain time (forexample, from one to two seconds) has elapsed, air-fuel ratio feedbackcorrection factors KO2 are calculated for the left and right banks fromoutputs of the PO2 sensors 50R, 50L on the left and right banks. Notethat a correction factor calculated from an output of the PO2 sensor onthe right bank is denoted as KO2R1 and that a correction factorcalculated from an output of the PO2 sensor on the left bank is denotedas KO2L1. The time then is denoted as tn.

[0057] Next, the flow proceeds to S18, where air-fuel ratio feedbackcorrection factors KO2 are calculated again for the left and right banksfrom outputs of the PO2 sensors 50R, 50L on the left and right banks ata time tn+m that is a time that results after a predetermined period oftime m has elapsed since the time tn. A correction factor for the rightbank calculated then is denoted as KO2R2 and that one for the left bankcalculated then is denoted as KO2L2.

[0058] Next, the flow proceeds to S20, as shown in the drawing, whetheror not differences exceed a predetermined value is determined which areobtained by subtracting the air-fuel ratio correction factors KO2R1,KO2L1 calculated at the time tn, respectively, from the air-fuel ratiofeedback factors KO2R2, KO2L2 calculated at the time tn+m that is thetime that results after the predetermined period of time m has elapsedsince the time tn.

[0059] If acknowledged in S20, the flow proceeds to S22, where the airsupply pipe 32 is determined (detected) to be normal, whereas if denied,or, to be more accurate, if denied regarding either or both of the KO2R,KO2L, the flow proceeds to S24, where the relevant air supply pipe 32Ror 32L on the left and right banks is determined (detected) to fail.

[0060] Here, a description will be made by reference to FIGS. 4 and 5.

[0061] If the exhaust secondary air supply system 30 is normal, in FIG.4, assuming that the air pump 34 starts driving at a time ta, the sameamount of air is supplied to the exhaust manifolds 22 on the left andright banks and then flows through the exhaust pipes 24, and as a resultof this, exhaust emissions get lean gradually at the positions where thePO2 sensors 50R, 50L are disposed. Consequently, values of the air-fuelratio feedback correction factors KO2R, KO2L that are calculated basedon detected values also gradually change so as to be corrected toward arich direction, and differences between values at the time tn that is atime resulting immediately after the air pump is driven and values atthe time tn+m are expanded to or over a certain value in conjunctionwith an increase in air amount during that time.

[0062] On the other hand, when the same amount of air is not supplied tothe exhaust manifolds 22 on the left and right banks due to theoccurrence of a failure of air leakage from either of the air supplypipes 32R, 32L on the left and right banks that is caused by a damagesuch as a crack made to the relevant air supply pipe 32R or 32L, theamount of air that flows through the exhaust pipes 24 comes to differbetween the left and right banks.

[0063] For example, as shown in FIG. 5, when a failure such as one thathas just been described above, the value of the air-fuel ratio feedbackcorrection factor calculated based on the detected value of the PO2sensor 50R does not show a predetermined change between the time tn andthe time tn+m as shown in the same drawing.

[0064] Consequently, by setting the predetermined value appropriatelyand comparing the air-fuel ratio feedback correction factor calculatedat the time tn and the air-fuel ratio feedback correction factorcalculated at the time tn+m which is the time resulting after thepredetermined period of time m has elapsed since the time tn for theleft and right banks, it can be determined that a failure occurs in theexhaust secondary air supply system 30, the failure including, to bemore accurately, those in which a damage such as a crack is caused tothe air supply pipe 32R or 32L on the side where the difference does notexceed the predetermined value, the sealing at the connecting portionbetween the flange portion 32 b of the relevant air supply pipe 32R or32L and the corresponding exhaust manifold 22 is insufficient or thesealing at the connecting portion between the air pump 34 or the cut-offvalve 36 and the air supply pipe 32 is insufficient.

[0065] Note that, here, the predetermined period of time is set based ona time needed for a change in operating conditions of the engine causedby an activation of the exhaust secondary air supply system 30 to cometo an end. Namely, since the air pump 34 of the exhaust secondary airsupply system 30 is driven by the electric motor 40, when the air pump34 is driven, the operating conditions such as load of the engine 10varies transiently due to a drastic increase in electric load. As aresult, there is caused a risk that the behaviors of the air-fuel ratioare affected. To deal with this, the predetermined period of time isdetermined to be set based on the time needed for the change in theoperating conditions to come to an end, or to be more specific, to beset to a time that is longer than the time needed for the change to cometo an end. Therefore, the failure of the exhaust secondary air supplysystem 30 can be detected more accurately. Note that the air pump 34 isstopped after a second calculation has been completed.

[0066] Since the failure detecting apparatus according to thisembodiment is constructed as has been described heretofore, the failureof the exhaust secondary air supply system 30 or, to be more specific, afailure such as a damage to the air supply pipe 32 can be detected moreeasily and accurately.

[0067]FIG. 6 is a flowchart illustrating an operation of detecting thefailure of the exhaust secondary air supply system 30 according to asecond embodiment of the invention.

[0068] To describe the flowchart, after processes from S100 to S106which are similar to those of the first embodiment have been performed,the flow proceeds to S108, where air-fuel ratio feedback correctionfactors KO2 are calculated again from outputs of the PO2 sensors 50R,50L of the left and right banks at a time tn+o which is a time resultingafter a time o has elapsed since the time tn. As this occurs, acorrection factor calculated for the right bank is denoted as KO2R2, andone for the left bank is denoted as KO2L2.

[0069] Next, the flow proceeds to S110, where air-fuel ratio feedbackcorrection factors KO2 are calculated again for the left and rightbanks, respectively, from outputs of the PO2 sensors 50R, 50L of theleft and right banks at a time tn+o+p which is a time resulting after acertain time p has elapsed since the time tn+o. A correction factorcalculated for the right bank then is denoted as KO2R3 and one for theleft bank is denoted as KO2L3. Note that the certain times o and p maybe of the same length as that of the predetermined time m or may be atime which is shorter than the time m.

[0070] Next, the flow proceeds to S112, where, as shown in the drawing,it is determined whether or not differences exceed a predeterminedvalue, respectively, which are obtained by subtracting the air-fuelfeedback correction factors KO2R1, KO2L1 which result at the time tnfrom the air-fuel feedback correction factors KO2R2, KO2L2 which resultat the time tn+o which is the time resulting after the predeterminedperiod of time o has elapsed.

[0071] If denied in S112, or to be more accurate, if denied regardingeither or both of KO2R, KO2L, the flow proceeds to S114, where the airsupply pipe 32R or 32L on the relevant bank of the left and right banksis determined (detected) to fail, whereas if acknowledged in S112, theflow proceeds to S116, where it is determined whether or not differencesexceed another predetermined value, respectively, which are obtained bysubtracting the air-fuel feedback correction factors KO2R1, KO2L1 whichresult at the time tn from the air-fuel feedback correction factorsKO2R3, KO2L3 which result at the time tn+o+p which is the time resultingafter the predetermined period of time o+p has elapsed.

[0072] If acknowledged in S116 as well, then, the flow proceeds to S118,where the air supply pie 32 is determined (detected) to be normal,whereas if denied, or, denied regarding either or both of KO2R, KO2L,the flow proceeds to S114, the air supply pipe 32R or 32L on therelevant bank of the left and right banks is determined (detected) tofail.

[0073] Note that, in the above process, differences between outputs ofthe PO2 sensors 50R, 50L at the time tn and outputs thereof at the timetn+m may be obtained so that the differences so obtained are comparedwith the predetermined value that is set appropriately so as to detect afailure. In the specification, “Values obtained based on outputs of theair-fuel sensor” is meant to include not only air-fuel ratio feedbackcorrection factors but also outputs themselves of the sensor.

[0074] In addition, while the predetermined value (and the secondpredetermined value) are set by selecting appropriately values that aregood enough to determine a failure from the differences as is describedabove, for example, a difference between outputs of the PO2 sensors 50R,50L when exhaust secondary air is not supplied may be learned, so thatthe predetermined value so set is corrected according to the learnedvalue.

[0075] Additionally, While the time tn and the time tn+m that is thetime resulting after the predetermined period of time has elapsed sincethe time tn are used for the air-fuel feedback correction factors KO2R,KO2L of the left and right banks, different times may be used betweenthe left and right banks. Namely, the time tn and the time tn+m are usedfor the right bank, while a time tq and a time tq+r that is a timeresulting after a predetermined period of time r has elapsed since thetime tq may be used for the left bank.

[0076] As has been described heretofore, according to the embodiments,there is provided the failure detecting apparatus for the exhaustsecondary air supply system 30 for supplying exhaust secondary air tothe position upstream of the catalytic converter 26 disposed along theexhaust system of the internal combustion engine (engine) 10. Thefailure detecting apparatus comprises the air-fuel ratio sensors (thePO2sensors 50R, 50L) for producing outputs corresponding to the densityof oxygen contained in exhaust gas flowing through the exhaust system,and the failure detecting section (the ECU 54, from S10 to S24) forcomparing values obtained at the different times, that is, the time tnand the time tn+m based on the outputs of the air-fuel ratio sensorswhen the exhaust secondary air is supplied so as to detect the failureof the exhaust secondary air supply system.

[0077] To be more specific, the failure detecting section includes thecorrection factor difference calculating section (the ECU 54, from S16to S18) for calculating the differences between the air-fuel feedbackcorrection factors KO2R, KO2LKO2R1, KO2L1 calculated based on theoutputs of the air-fuel ratio sensors at the time tn and the air-fuelfeedback correction factors KO2R, KO2LKO2R2, KO2L2 calculated at thetime tn+m that is the time that results after the predetermined periodof time m has elapsed since the time tn and the comparing section (theECU 54, S20) for comparing the differences so calculated with thepredetermined value. Therefore, when the calculated differences do notexceed the predetermined value, the exhaust secondary air supply systemis detected to fail.

[0078] In addition, the predetermined period of time m is set based onthe time needed for the change in the operating conditions of theinternal combustion engine caused by an activation of the exhaustsecondary air supply system to come to an end.

[0079]FIG. 7 is a flowchart illustrating an operation of detecting thefailure of the exhaust secondary air supply system 30 according to athird embodiment of the invention.

[0080] To start the description of the flowchart, in step S10′, whetheror not it is in a monitor area (a failure detecting area) is determined.It is determined to be in a monitor area when the engine 10 is in theidling state or other steady-state operating conditions after the engine10 is started and the warming up thereof is completed.

[0081] If denied in step 10′, processes in the following steps areskipped, whereas if acknowledged, the flow proceeds to step S12′, wherewhether or not the failure of the exhaust secondary air supply system 30has been detected is determined. If acknowledged in step S12′, too,processes in the following steps are skipped. Note that the processes inthe following steps are also skipped, for example, if it is determinedthat the electric motor 40 does not fail but is heated excessively dueto the excessive energization of the electric motor 40.

[0082] On the other hand, if denied in step S12′, the flow proceeds toS14′, where the air pump 34 is switched on, or the electric motor 40 isenergized so as to activate the air pump 34, and the learning of theair-fuel feedback correction factor KO2 is prohibited. Namely, in ordernot to effect the originally desired air-fuel ratio feedback controlthrough the artificial operation of air-fuel ratio for failuredetection, the learning thereof is prohibited.

[0083] Next, the flow proceeds to S16′, air-fuel feedback correctionfactors KO2 are calculated from outputs of the PO2 sensors 50R, 50L ofthe left and right banks. Note that a correction factor calculated froman output of the PO2 sensor on the right bank is denoted as KO2R and acorrection factor calculated from an output of the PO2 sensor on theleft bank is denoted as KO2L. Next, maximum values KO2RMAX and KO2LMAXare obtained, respectively.

[0084] Next, proceed to S18′, the maximum value of KO2LMAX so obtainedis then subtracted from the maximum value of KO2RMAX so obtained, and adifference Δmax is obtained in an absolute value.

[0085] Next, proceed to S20′, where the difference Δmax obtained in anabsolute value is then compared with a predetermined value, and whetheror not the difference Δmax exceeds the predetermined value isdetermined. If denied, then the flow proceeds to S22′, where the airsupply pipe 32 is determined (detected) to be normal, whereas ifacknowledged, the flow proceeds to S24′, where the air supply pipe 32 isdetermined (detected) to fail.

[0086] Here, to describe by reference to FIGS. 4 and 5, in the eventthat the exhaust secondary air supply system 30 is normal, assuming thatthe air pump 34 is started to be driven at a time ta in FIG. 4, the sameamount of air is supplied to the exhaust manifolds 22 of the left andright banks and then flows through the exhaust pipes 24, as a result ofwhich, exhaust gases become gradually lean at positions where the PO2sensors 50R, 50L are disposed. Consequently, while values of air-fuelratio feedback correction factors KO2R, KO2L that are calculated basedon the values so detected change gradually as shown in the drawing tocorrect the lean exhaust gases to a rich direction, a difference betweenthe maximum values becomes 0 or a minute number.

[0087] On the other hand, in the event that the same amount of air isnot supplied to the exhaust manifolds 22 of the left and right banks dueto there occurring a failure in which a damage such as a crack is causedin either of the air supply pipes 32R, 32L on the left and right banksto thereby cause a leakage of air therefrom, the amount of air flowingthrough the exhaust pipes 24 becomes difference between the left andright exhaust pipes.

[0088] For example, as shown in FIG. 5, assuming that a failure like onedescribed above occurs in the air supply pipe on the right bank 10R,since the amount of air supplied becomes short, a change in value of theair-fuel ratio feedback correction factor KO2R calculated based on adetection value of the PO2 sensor 50R becomes smaller than a change inthe air-fuel ratio feedback correction factor KO2L on the left bank 10L,and a difference between the two factors increases gradually to bemaximum with the maximum value KO2LMAX.

[0089] Consequently, by comparing the difference Δmax between theair-fuel ratio feedback correction factors KO2RMAX and KO2LMAX with thepredetermined value while changing the predetermined valueappropriately, it is possible to determine that there has occurred afailure in the exhaust secondary air supply system 30, the failure beingdefined more accurately as a failure in which a damage such as a crackis caused in the air supply pipe 32 which has the smaller maximum value(the smaller change in air-fuel ratio feedback correction factor),sealing at the connecting portion between the air pump 34, sealing atthe connecting portion between the flange portion 32 b of the air supplypipe 32 and the exhaust manifold 22 is insufficient, or the cut-offvalve 36 and the air supply pipe 32 is insufficient.

[0090] Since the failure detecting apparatus according to the thirdembodiment of the invention is constructed as has been described above,the failure of the exhaust secondary air supply system 30, or to be morespecific, a failure such as a damage to the air supply pipe can bedetected easily and accurately.

[0091] Note that while, as has been described above, the predeterminedvalue is set by selecting appropriately a value which is good enough todetermined a failure from the difference, for example, a differencebetween outputs of the PO2 sensor 50R, 50L which results when exhaustsecondary air is not supplied may be learned, so that the predeterminedvalue set in accordance with the learned value may be corrected.

[0092] Note that while, in the above configuration, the failure isdetected by obtaining the maximum values KO2MAX, KO2LMAX of the air-fuelratio feedback correction factors of the left and right banks and thencomparing the difference Δmax between the maximum values with thepredetermined value, the maximum values are not necessarily obtained inan strict fashion. For example, a maximum value may be obtained for oneof the air-fuel feedback correction factors KO2R, KO2L, while a valuejust prior to the maximum value may be used for the other to calculate adifference between the two feedback correction factors for comparisonwith the predetermine value. Alternatively, values just prior to themaximum values may be used for both of the air-fuel feedback correctionfactors KO2R, KO2L to calculate a difference between the two feedbackcorrection factors for comparison with the predetermine value.

[0093] Furthermore, a difference between outputs (or maximum values) ofthe PO2sensors 50R, 50L is obtained, and the difference may be comparedwith the predetermined value which is set appropriately so as to detecta failure. In this case, when the difference exceeds the predeterminedvalue, a value toward the rich direction is outputted or the air supplypipe 32 on the PO2 sensor 50R or 50L side which indicates a reversal ofoutput is regarded as failing.

[0094] As has been described above, according to the third embodiment,there is provided the failure detecting apparatus for an exhaustsecondary air supply system 30 for supplying exhaust secondary air to aposition upstream of the catalytic converter 26 disposed along theexhaust system (the exhaust manifolds 22, the exhaust pipes 24) of theinternal combustion engine (the engine) 10. The failure detectingapparatus comprises a plurality of air supply pipes 32R, 32L connectedto the exhaust system (the exhaust manifolds 22) of the internalcombustion chamber for supplying the exhaust secondary air thereto,respectively, a plurality of air-fuel ratio sensors (the PO2 sensors50R, 50L) disposed along the exhaust system (the exhaust pipes 24) atpositions downstream of the plurality of air supply pipes for producingan output corresponding to the density of oxygen contained in exhaustemissions flowing through the exhaust system, respectively, and thefailure detecting section (ECU 54, from S10 to S24) for comparing valueswith each other which are obtained based on the outputs of the pluralityof air-fuel ratio sensors so as to detect a failure of the exhaustsecondary air supply system.

[0095] To be more specific, the failure detecting section comprises thecorrection factor difference calculating section (ECU 54, S16, S18) forcalculating a difference (to be more accurate, the difference Δmaxbetween the maximum values KO2RMAX and KO2LMAX) between air-fuel ratiofeedback correction factors KO2R, KO2L calculated, respectively, basedon the outputs of the plurality of air-fuel ratio sensors, and thecomparing section (ECU 54, S20) for comparing the difference socalculated with the predetermined value. Therefore, when the calculateddifference exceeds the predetermined value, the exhaust secondary airsupply system is detected to fail.

[0096] In addition, the internal combustion engine is a V-type engine,the single air pump is provided, and the air pipe 32R, 32L is connectedto the air pump and is branched at an intermediate position along thelength thereof so as to be connected to the banks 10R, 10L of the V-typeengine, respectively.

[0097] Note that while the first to third embodiments have beendescribed by taking for example the construction in which the air supplypipes 32R, 32L are disposed on the left and right banks 10R, 10L of theV-type engine, respectively, and the PO2 sensors 50R, 50L are disposeddownstream of the air supply pipes, respectively, the invention is notlimited thereto and therefore equally applies to any engines other thanV-type ones provided that the engine has a plurality of systems in itsexhaust system with air supply pipes and air-fuel ratio sensors beingdisposed on the respective exhaust systems, respectively. In addition,while the O2 sensors are used as the air-fuel ratio sensors, theinvention is not limited thereto and therefore any type of sensors maybe used which is adapted for producing an output in proportion to thedensity of oxygen.

[0098] According to the first aspect of the invention, the failure ofthe exhaust secondary air supply system can be detected easily andaccurately. In addition, since the necessity is obviated of doing thetroublesome work in which the output of the air-fuel ratio sensorresulting when the exhaust secondary air supply system is normal islearned to be verified, there is no risk that the detection of thefailure of the exhaust secondary air supply system is delayed.Furthermore, sine the failure detection is not such as to be implementedbased on the output of the air-fuel ratio sensor when exhaust secondaryair is supplied, in other words, based on the output of the air-fuelratio sensor when the exhaust secondary air supply system is activatedintermittently so as to supply exhaust secondary air intermittently,there is no risk that an erroneous failure detection results from theeffect of an error caused by a difference in load of the internalcombustion engine.

[0099] According to the second aspect of the invention, the failure ofthe exhaust secondary air supply system can be detected more easily andaccurately by setting the predetermined period of time appropriately orto, for example, a time that will be described below as a time neededfor a change in the operating conditions such as load of the internalcombustion engine caused by an activation of the exhaust secondary airsupply system to come to an end.

[0100] According to the third aspect of the invention, since thepredetermined period of time is set based on a time needed for a changein the operating conditions of the internal combustion engine caused byan activation of the exhaust secondary air supply system to come to anend, even when the air-fuel ratio sensor exhibits unexpected behaviorsaccording to the change, an effect resulting from the unexpectedbehaviors can be avoided, whereby the failure of the exhaust secondaryair supply system can be detected more easily and accurately.

[0101] According to the fourth aspect of the invention, the failure ofthe exhaust secondary air supply system can be detected easily andaccurately. In addition, the necessity is obviated of doing thetroublesome work in which the output of the air-fuel ratio sensorresulting when the exhaust secondary air supply system is normal islearned to be verified, and there is caused no inconvenience that thedetection of the failure of the exhaust secondary air supply system isdelayed. In addition, in a case where an air supply pipe is provided foreach exhaust system of an engine such as a V-type engine which has anexhaust system on each bank, it is possible not only to detect theexistence of a failure but also to detect which air supply pipe on thebanks suffers from the failure when the failure is detected.

[0102] According to the fifth aspect of the invention, the failure ofthe exhaust secondary air supply system can be detected more easily andaccurately.

[0103] According to the sixth aspect of the invention, in a V-typeengine in which an air supply pipe is connected to each of banksthereof, the failure of an exhaust secondary air supply system can bedetected easily and accurately.

What is claimed is:
 1. A failure detecting apparatus for an exhaustsecondary air supply system for supplying exhaust secondary air to aposition upstream of a catalytic converter disposed along an exhaustsystem of an internal combustion engine, the failure detecting apparatuscomprising: an air-fuel ratio sensor for producing an outputcorresponding to a density of oxygen contained in exhaust gas flowingthrough the exhaust system; and a failure detecting section forcomparing values obtained at different times based on outputs of theair-fuel ratio sensor when the exhaust secondary air is supplied so asto detect a failure of the exhaust secondary air supply system.
 2. Afailure detecting apparatus for an exhaust secondary air supply systemas set forth in claim 1, wherein the failure detecting section comprisesa correction factor difference calculating section for calculating adifference between an air-fuel feedback correction factor calculatedbased on an output of the air-fuel ratio sensor at a time tn and anair-fuel feedback correction factor calculated at a time tn+m that is atime that results after a predetermined period of time m has elapsedsince the time tn and a comparing section for comparing the differenceso calculated with a predetermined value, so as to determine that theexhaust secondary air supply system fails when the calculated differencedoes not exceed the predetermined value.
 3. A failure detectingapparatus for an exhaust secondary air supply system as set forth inclaim 2, wherein the predetermined period of time is set based on a timeneeded for a change in operating conditions of the internal combustionengine caused by an activation of the exhaust secondary air supplysystem to come to an end.
 4. A failure detecting apparatus for anexhaust secondary air supply system for supplying exhaust secondary airto a position upstream of a catalytic converter disposed along anexhaust system of an internal combustion engine, the failure detectingapparatus comprising: a plurality of air supply pipes connected to theexhaust system of the internal combustion engine for supplying theexhaust secondary air, respectively; a plurality of air-fuel ratiosensors disposed along the exhaust system at positions downstream of theplurality of air supply pipes for producing an output corresponding to adensity of oxygen contained in exhaust emissions flowing through theexhaust system, respectively; and a failure detecting section forcomparing values with each other which are obtained based on the outputsof the plurality of air-fuel ratio sensors so as to detect a failure ofthe exhaust secondary air supply system.
 5. A failure detectingapparatus for an exhaust secondary air supply system as set forth inclaim 4, wherein the failure detecting section comprises a correctionfactor difference calculating section for calculating a differencebetween air-fuel ratio feedback correction factors calculated,respectively, based on the outputs of the plurality of air-fuel ratiosensors, and a comparing section for comparing the difference socalculated with a predetermined value, so as to determine that theexhaust secondary air supply system fails when the calculated differenceexceeds the predetermined value.
 6. A failure detecting apparatus for anexhaust secondary air supply system as set forth in claim 4, wherein theinternal combustion engine is a V-type engine, wherein the failuredetecting apparatus comprises an air pump, and wherein the air supplypipe is connected to the air pump and is branched at an intermediateposition along its length so as to be connected to banks of the V-typeengine, respectively.
 7. A failure detecting apparatus for an exhaustsecondary air supply system as set forth in claim 5, wherein thecorrection factor difference calculating section calculates a differencebetween maximum values of air-fuel ratio feedback correction factorscalculated, respectively, based on the outputs of the plurality ofair-fuel ratio sensors.